1) Haalireader, http://haali.su/pocketpc/, fb2 & txt reader, fast, customizable, I have read lots of really big books on it. There is a companion desktop program to convert html/doc/rtf to fb2, any2fb http://www.gribuser.ru/xml/fictionbook/. Annotations, highlighting and double tapping dictionary pop-up available but in a special format.
2) Plucker and plucker desktop http://www.plkr.org/. Wonderful, with this you can download any website (recursion depth customizable, restrict to a domain also available) and convert it to a single pdb with links & images. The reader acts like a browser, you can follow the links and going back or forward. FYI, I have read the whole Solar System Wikipedia article (50MB) with this. There is another desktop program for pdb generation, sunriseXP (perhaps you know this :-) http://sunrise.mobileread.com/).
3) ubook http://www.gowerpoint.com/. Very nice also, the best: you can use it with Webster with just a bit of tweaking. Double tap, look up and that's all. And if you don't know the meaning of some words in the Webster definition of a word you can search again the same way. Then there are handy buttons to go back to the book that you are reading.

I've written a program for reading eBooks on Windows Mobile devices. It can read txt, html and (DRM-free) ePub files. Screenshots attached.
[updated: version 1.0 released]

The program is called 'Freda' - short for 'free reader', since it is free software (both in the sense of 'free beer', as I am not charging for it, and in the sense of 'free speech', as I am licensing my code under the GPL).

It's in 'public beta' at the moment, so please download it and let me have any feeback.

The program uses two third-party libraries, SharpZipLib and the Majestic-12 HTMLParser library; both are licensed on terms permitting their redistribution for commercial and non-commercial purposes. More information is here

Happy reading!

Thanks Jim, I am using less my HTC and more my prs but I have installed your soft and works very well, congratulations.

Okay, I tried it again. It was much slower than Mobipocket Reader at pagination, and turned pages slower as well.

My problem with adding uBook is that there are vast numbers of 7, 8, 9 year old devices out there that are still perfectly functional. uBook won't run well on them becuase of the older, weaker CPUs.

In ubook-options set colordepth from 24Bit to 8Bit. That boosts performance a lot - especially on the old Cassiopeias. With a good Font you can also turn of Fontsmoothing (i never used it anyway). I am still using ubook on a Dell X5-Pda.
Before that i used ubook it with Casiopeia E-15/E105/E115/E125 over serveral years. In my opinion it is still one of most flexible readersoftware out there when reading rtf/Html/prc. I switched to ubook because the newer Mobipockets had problems with Greyscale devices like the Casio E-15 . On this Device Mobipocket give you a Black in Black screen.
RTF also didn't work with Mobipocket.

There are many readers for Palm devices.
Depends on the device. I use Docs to Go which is in the T3's ROM and "the Reader" to read EPUB's.

For WM5, WM6(I updated the OS on my iPAQ 211) I use Freda reader.
It works well and reads HTML, EPUB and TXT files.
Many thanks to Jim Chapman.
I convert to EPUB and read on either device depending on how good my eyes are that day. I prefer the way HTML displays but use EPUB for both devices.
The choice is yours.
The Calibre viewer, NOT the main program, is easy to associate with various file types.
I just click on the file and it comes up in XP, Win 7 and Linux.
It handles almost anything but does not display 2 pages at once, or I have not discovered how to configure it that way yet.
Ain't it great to have choices??